[0001] The invention relates to a method and apparatus for detecting luminescent security
features in or on documents.
[0002] It has been proposed in the past to irradiate security documents, particularly paper
sheets such as bank notes, with ultra-violet radiation to determine whether or not
the document is luminescent. The generation of a bluish luminescence can be indicative
of a forged document. Recently, luminescent materials have been incorporated into,
or printed on, documents to provide security features in themselves. One form of security
feature is a luminescent thread. Previous methods for detecting such security features
involve using accurate and complex filtering techniques to detect whether luminescence
emitted by the document falls within a particular wavelength band. These methods do
not enable security features which luminesce at substantially the same wavelength
as other features to be discriminated.
[0003] In accordance with one aspect of the present invention, a method of detecting luminescent
security features in or on documents comprises illuminating a test document with an
interrogation beam; sensing luminescence emitted from the document with sensing means;
dividing the area over which luminescence is sensed into a plurality of portions;
comparing the intensity of the luminescence of each portion with the average intensity
of all the portions; and detecting the presence of a security feature if the two intensities
satisfy a predetermined realtionship.
[0004] We also provide in accordance with a second aspect of the present invention a detector
for detecting luminescent security features in or on documents, the detector comprising
means for illuminating a test document with an interrogation beam; sensing means for
sensing luminescence emitted from the document; and discriminating means for discriminating
between luminescence due to a security feature and other luminescence emitted by the
document, the discriminating means comprising means for comparing the average intensity
of the luminescence emitted over a plurality of adjacent portions of the document
with the intensity of each portion and detecting means for detecting whether the compared
intensities satisfy a predetermined relationship.
[0005] With this method and apparatus, we are able to detect luminescent security features
even where the luminescence of the security feature has substantially the same wavelength
as other parts of the documents.
[0006] The relationship between the two intensities may be for example a minimum ratio which
must be achieved or possibly the two intensities could be subtracted one from the
other and the presence of the security feature detected if the difference exceeds
a predetermined threshold.
[0007] In one example, the method may further comprise repeating the sensing step over a
plurality of areas of the document, and detecting the presence of a composite security
feature if a predetermined arrangement of individual security features are detected.
Such a method is particularly suitable for detecting a luminescent security thread,
the method comprising feeding a test document past the sensing means in a direction
such that a security thread will extend in the feed direction; sensing luminescence
emitted from a plurality of sets of adjacent portions of the test document, each set
of adjacent portions being aligned transversely to the security thread; determining
which portion or portions of each set of portions has emitted luminescence corresponding
to a security feature; and detecting the presence of a security thread if security
features are detected for each set of portions in positions which are arranged in
a manner corresponding to a security thread. Such an arrangement could be a straight
line.
[0008] Preferably, the document containing the security thread passes the sensing means
with the thread extending parallel to the direction of motion.
[0009] If a minimum number of individual security features, representing successive portions
of the security thread, are detected then a composite security feature comprising
the whole thread is detected.
[0010] Conveniently, the method further comprises periodically checking for failure of the
sensing means and this may be carried out between successive documents. This enables
the method to be substantially automated since the checking step will reveal any failure
in the sensing means and such failure can be indicated by means of a suitable alarm.
[0011] The checking step may be carried out by checking means which preferably comprises
means for ensuring that no luminescence is sensed when no document is being illuminated;
and means for illuminating the sensing means and means to check that the sensing means
then senses the apparent presence of luminescence.
[0012] In one example, the checking means includes circuitry for ensuring that the output
from the sensing means does not exceed a low level threshold when no document is present.
The means for illuminating the sensing means may be a light emitting diode (LED).
[0013] It is possible that a forger might try to duplicate a luminescent security feature
normally incorporated into the document by providing a similar feature on a surface
of the document. In order to detect such a forgery, the apparatus preferably further
comprises another sensing means positioned on the opposite side of the document to
the one sensing means to detect luminescence emitted from that side of the document.
[0014] With this arrangement, if the forged feature is on the side of the document which
is illuminated it will strongly luminesce on that side while relatively weak luminescence
will be detected on the other side of the document. In contrast, a genuine security
feature within the document will strongly luminesce on both sides of the document.
[0015] Preferably, the or each sensing means comprises a photo-diode array. This is particularly
suitable for dividing the area over which luminescence is sensed into a plurality
of portions. Conveniently, where the detector is arranged to detect a security thread,
the or each sensing means comprises a line of sensors, the line being transverse,
in use, to the security thread.
[0016] One important advantage of the detector according to the second aspect of the invention
is that it may be used in apparatus for automatically detecting the presence of security
features on a plurality of documents. In this case, means are provided for automatically
feeding successive ones of the documents past the illuminating means and the sensing
means; together with means for indicating a document in which a security feature has
not been detected.
[0017] The detector is capable of detecting security features very rapidly and this enables
high speed operation to be underaken (e.g. 20 bank notes/sec).
[0018] In one convenient form, the detector includes checking means as outlined above and
further comprises means for indicating the presence of a fault in the or each sensing
means. This is particularly useful for fully automated operation since the indicating
means can for example generate a fault signal for terminating operation and/or initiating
an alarm. Conveniently, the checking means is operable between successive documents
so that after each document is processed a check is carried out.
[0019] The form of the interrogation beam depends on the nature of the luminescent security
feature but is typically white light causing the luminescent security feature to luminesce
in the infra-red. With a suitable choice of filters and light source, security features
luminescing in other regions of the spectrum may be detected, when illuminating with
other than visible light. Furthermore, by arranging to sense luminescence a predetermined
time interval after the document has been illuminated by the interrogation beam, or
by suitably modulating the interrogation beam and including de-modulating circuitry
in the processing circuitry, it is possible to differentiate between fluorescence
and phosphorescence.
[0020] Two examples of methods and detectors in accordance with the present invention will
now be described with reference to the accompanying drawings, in which:-
Figure lA is a plan of a document incorporating a security thread;
Figure 1B is a diagrammatic view of a detector;
Figure 2 illustrates circuitry for use with the detector of Figure 1B;
Figures 3A, 3B, 3C are diagrammatic cross-sections through bank notes illustrating
how forged and genuine security features are distinguished;
Figure 4 is a diagrammatic illustration of apparatus for use in detecting the forged
documents shown in Figures 3B and 3C; and,
Figure 5 illustrates additional circuitry for use when detecting phosphorescent security
features.
[0021] A bank note 1 having a rectangular shape is illustrated in Figure 1A. A luminescent
security thread 2 extends across the width of the bank note 1. In this case, the security
thread 2 is incorporated within the paper of the bank note.
[0022] A detector for detecting the security thread 2 is illustrated in Figure 1B. The detector
may be incorporated in automatic bank note sorting apparatus (not shown). The detector
comprises a housing 3 of circular cross-section one end 4 of which is closed while
a filter 5 is mounted, at the other end. The filter 5 transmits, in this case, infra-red
luminescence but will be chosen in general to match the characteristics of the security
features to be detected in order to filter out unwanted wavelengths. A lens 6 is mounted
in the housing 3 adjacent the filter 5 and is arranged to focus luminescence passing
through the filter 5 onto a photo-diode array 7. The photo-diode array 7 is mounted
on a support 8 secured within the housing 3. The photo-diode array 7 is electrically
connected to circuitry on a printed circuit board 9 also mounted (by means not shown)
in the housing 3. The circuitry is illustrated in Figure 2.
[0023] A light emitting diode (LED) 10 is mounted in an outwardly extending support 11 fixed
in an aperture 12 in the housing 3. The LED 10 is arranged to direct light onto the
photo-diode array 7 when it is illuminated. The detector also includes a tungsten
halogen light source 13 mounted to another housing part (not shown) which is arranged
to transmit visible, white light through an infra-red absorbing filter 14 into a fibre
optic light guide 15. The fibre optic light guide 15 is mounted to the housing 3 by
a suitable clamp 16. The fibre optic light guide 15 terminates in a fishtail portion
17 which directs light onto an area 18.
[0024] In use, documents 1 are fed in succession by a conventional means such as conveyor
belts (not shown) under the housing 3 and the light guide 15 in the direction of the
arrow 19. The documents 1 are fed so that the security thread 2 is parallel with the
direction of motion. The photo-diode array 7 comprises a single line of photo-diodes
arranged transversely to the direction of movement of the document. Light from the
light source 13 is first filtered to remove infra-red wavelengths by the filter 14
and is then directed via the fibre optic fishtail 17 onto the document 1. A strip
of the illuminated surface of the document 1 is then imaged onto the photo-diode array
7 by the lens 6 and the infra-red transmitting filter 5 removes all visible wavelengths
from the light beam and thus prevents any reflected light from reaching the array
7. Thus, no light reaches the photo-diode array 7 unless a luminsecent feature on
or in the document is stimulated by the incident visible light to emit infra-red radiation.
In practice, a small amount of light of the unwanted wavelengths may be transmitted
by the filters 5, 14, so that a small signal may be generated by the elements of the
array 7 but this can be dealt with by the circuitry to be described.
[0025] The width of document to be interrogated is chosen as appropriate depending on the
expected position of the security thread 2. The document 1 is scanned as it passes
beneath the lower end of the housing 3 and a typical interval between scans is 1 mm.
This may be adjusted to suit specific documents.
[0026] The image of the security thread 2, when focused onto the photo-diode array 7, will
cause one (or an adjacent pair) of the photo-diode outputs to be significantly different
from all the others. It is this property which is looked for when the signals are
processed.
[0027] The signal processing circuitry is illustrated in Figure 2. In order to reduce the
complexity of the processing electronics, the outputs of the diode array 7 are sequentially
multiplexed by a multiplexer 20 into a single analogue datastream. The outputs from
the photo-diode array 7 are also fed to a circuit 21 for obtaining the means output
and from there to a variable resistance 22 which feeds a preset fraction of the mean
output to a comparator 23. The single analogue datastream output from the multiplexer
20 is also fed to the comparator 23 so that a comparison is made between each photo-diode
output and the mean of the entire array. The result of this comparison, now a digital
signal, is auto-correlated. Auto-correlation is achieved by taking the digital signal
from the comparator 23 and feeding this to a delay circuit 24 and to an AND gate 25.
The output of the delay circuit 24 is then fed to the AND gate 25 where it is ANDed
with the undelayed signal to produce the desired auto-correlation. The auto-correlated
signal from the AND gate 25 is then counted by a counter 26 and if the counter output
exceeds a preset value, the security thread 2 is deemed to be detected and a latch
27 is set.
[0028] If a scan is carried out at 1 mm intervals as indicated previously, the counter 26
will be set to count a sufficient number of scans for all or a large portion of the
security thread to be scanned.
[0029] Control electronics 28 of conventional form (such as a microprocessor) is also provided
on the PCB 9 to carry out two checks on the diodes of the photo-diode array 7. Firstly,
the control electronics looks momentarily at the outputs of the photo-diodes in the
array 7 between the passage of successive documents 1 (when light from the source
13-will not be reflected through the filter 5) to ensure that the output signal of
each diode is below a low level threshold setting. Secondly, the LED 10 is momentarily
activated between the passage of successive documents and the control electronics
28 checks the outputs of each diode of the photo-diode array 7 to ensure that each
has an output that exceeds a high level threshold. If a fault is detected during the
tests the control electronics 28 provides a suitable output signal.
[0030] Figure 3A illustrates a document 1 having a security thread 2 incorporated within
the document. When such a document 1 passes beneath the detector illustrated in Figure
1B, strong luminescence (L) is emitted on both sides of the document 1, as indicated
by the long arrows in Figure 3A. If, however, a document having a luminescent security
thread or luminescent ink on its surface passes beneath the detector of Figure 1B,
the intensity of luminescence (L) emitted on each side of the document will differ.
Figure 3B illustrates the case where a security thread 2 is provided on the surface
which is illuminated and in this case strong luminescence occurs from that surface
but only a weak luminescence (L) indicated by the relatively short arrow will be detected
from the other surface. Conversely, Figure 3C illustrates the same document 1 of Figure
3B but after having been reversed when only weak luminscence (L) will occur from both
sides of the document 1.
[0031] Figure 4 illustrates diagramatically another example of a detector in this case for
discriminating between a document 1 having an internal security thread 2 as shown
in Figure 3A and a document having a security thread 2 on its surface. The apparatus
of Figure 4 is essentially the same as that shown in Figure 1B but with the addition
of a second housing 3' having exactly the same components as the housing 3 but positioned
on the opposite side of the document 1 to the housing 3. Additional illumination means
are not, however, provided. The photo-diode array (not shown) supported by the housing
3' will detect luminenscence emitted from the adjacent side of the document 1 and
by comparing the intensities of luminescence detected by each photo-diode array using
suitable electronic circuitry (not shown) the authenticity of the security thread
2 can be determined. In other words, a document will only be classified as genuine
if luminescence of sufficient strength is sensed by both photo-diode arrays.
[0032] The output from the latch 27 is fed to the control electronics 28 which provides
an ouput indicating the presence or absence of a security feature in or on the document.
[0033] Where it is desired to discriminate between phosphoresecent and fluorescent features,
the apparatus of Figure 1B may be used but with the modified circuitry shown in Figure
5. In this case, the interrogation beam supplied by the source 13 is modulated at
a frequency F and the output from the photo-diode array 7 is fed to a phase sensitive
detector 29 to which is also supplied the frequency F. The output from the phase sensitive
detector 29 is fed via a low pass filter 30 to the multiplexer 20 and the circuit
21 shown in Figure 2. Since the interrogation beam supplied by the source 13 is not
continuous it is possible to distinguish between fluorescent and phosphorescent features
using the circuitry shown in Figure 5.
[0034] In an alternative arrangement (not shown) the area 18 which is illuminated may be
positioned upstream from the position shown in Figure 1B so that luminescence is received
by the diode array 7 a predetermined time interval after the test document has been
illuminated with the interrogation beam so that fluorescent and phosephorescent features
may be differentiated.
1. A method of detecting luminescent security features in or on documents (1), the
method comprising illuminating a test document with an interrogation beam; sensing
luminescence emitted from the document with sensing means (7); dividing the area over
which luminescence is sensed into a plurality of portions; comparing the intensity
of the luminescence of each portion with the average intensity of all the portions;
and detecting the presence of a security feature (2) if the two intensities satisfy
a predetermined relationship.
2. A method according to claim 1, further comprising repeating the sensing step over
a plurality of areas of the document, and detecting the presence of a composite security
feature (2) if a predetermined arrangement of individual security features are detected.
3. A method according to claim 2 for detecting a luminescent security thread (2),
the method comprising feeding a test document (1) past the sensing means (7) in a
direction such that a security thread will extend in the feed direction (19); sensing
luminescence emitted from a plurality of sets of adjacent portions of the test document,
each set of adjacent portions being aligned transversely to the security thread; determining
which portion or portions of each set of portions has emitted luminescence corresponding
to a security feature; and detecting the presence of a security thread (2) if security
features are detected for each set of portions in positions which are arranged in
a manner corresponding to a security thread.
4. A method according to any of claims 1 to 3, further comprising periodically checking
for failure of the sensing means (7).
5. A method according to claim 4, wherein the checking step is carried out between
successive documents.
6. A method according to any of the preceding claims, wherein luminescence (L) emitted
from both sides of the document (1) is sensed.
7. A method according to any of the preceding claims, wherein the interrogation beam
is modulated, and the sensed luminescence is first demodulated prior to the comparing
step so that fluorescent and phosphorescent features may be differentiated.
8. A method according to any of claims 1 to 6, wherein the sensing means senses luminescence
emitted from the document a predetermined time interval after the test document has
been illuminated with the interrogation beam so that fluorescent and phosphorescent
features may be differentiated.
9. A method according to any of the preceding claims, wherein the predetermined relationship
comprises a minimum predetermined threshold which must be exceeded by the difference
between the two intensities.
10. A detector for detecting luminescent security features in or on documents (1),
the detector comprising means (13,15) for illuminating a test document (1) with an
interrogation beam; sensing means (7) for sensing luminescence emitted from the document
(1); and discriminating means (20-27) for discriminating between luminescence due
to a security feature and other luminescence emitted by the document, the discriminating
means comprising comparison means (23) for comparing the average intensity of the
luminescence emitted over a plurality of adjacent portions of the document with the
intensity of each portion and detecting means (26) for detecting whether the compared
intensities satisfy a predetermined relationship.
11. A detector according to claim 10, further comprising checking means (28,10) for
checking for failure of the sensing means.
12. A detector according to claim 11, wherein the checking means comprises means (28)
for ensuring that no luminescence is sensed when no document is being illuminated;
and means (10) for illuminating the sensing means (7) and means (28) to check that
the sensing means then senses the apparent presence of luminescence.
13. A detector according to any of claims 10 to 12, further comprising another sensing
means positioned on the opposite side of the document (1) to the one sensing means
(7) to detect luminescence emitted from that side of the document (1).
14. A detector according to any of claims 10 to 13, wherein the or each sensing means
comprises a photo-diode array (7).
15. A detector according to any of claims 10 to 14, for detecting a luminescent security
thread (2), wherein the or each sensing means comprises a line of sensors (7), the
line being transverse, in use, to the security thread.
16. Apparatus for automatically detecting the presence of security features in a plurality
of documents, the apparatus comprising a detector according to any of claims 10 to
15, means for automatically feeding successive ones of the documents past -the illuminating
means (13,15) and the sensing means (7); and means (28) for indicating a document
in which a security feature has not been detected.
17. Apparatus according to claim 16, when dependant on claim 10 or claim 11, further
comprising means (28) for indicating the presence of a fault in the or each.sensing
means.
18. Apparatus according to claim 17, wherein the checking means (28, 10) is operable
between successive documents.